I'm pretty darn careful about not hitting the front air dam components on my car but despite the care it seems almost inevitable. Also the parts down there can "hook" so that even more damage is done when you reverse... If you can see the corner air dam components (triangular ones) peeking into your wheel well and bowing the fender liner from the side then you too have damaged your parts. bmwfans.info has the costs for both the center piece and the two triangular side pieces at only ~$135. See parts 20 and 23 in image below. However, I suspect they would run at more like $200 from any dealer, maybe more depending on the gouge factor... Thus when doing a recent brake pad job I decided to tackle a repair of these parts to get them back to structurally sound and in the correct position.

I know, I know I should have taken pictures, this repair is pretty simple and anyone remotely handy can do this. I suspect I can get another couple of years from this before having to bite the bullet and actually buy new parts. Let me emphasize, despite this being somewhat of a hack, the work is absolutely structurally sound and invisible, even upon a close inspection on a rack. I'd almost guarantee you could return a lease car with this repair and have it go totally unnoticed. Similarly I would have no problem running the car to top speed with these repairs (i.e. full aero loading). This entire job can be done in 1-2 hours. It is much easier on jacks/jack stand.

#6 speed nuts (Only if you some fell off due to damage or you lost any, most any auto parts store will have these)

Hammer or vise

Details:

Remove parts: There are about 15-20 black hex head screws to remove under the bottom of the bumper cover and under the front/bottom of the wheel liners. Also remove a few of the lower wheel liner screws so those can be flexed out of the way and the parts tucked back in properly. To fully remove the center section reach inside and find the wire cable hangars (#24 below in pic), rotate the top captive "head" 90° and slide the hangars out of their slots. This is the only remotely tricky parts of the disassembly.

Patches: Use some sheet metal shears to cut patches from the 1/16" thick aluminum sheet. You will absolutely be able to see where your OEM parts have failed. It will be right on the mounting locations where the "speed nut" clips are located. My patches were typically about 3/4" x 1 1/4" but varied depending on what area I was working on. Cut the patches so you have plenty of remaining good material on each side to attach it to. Flatten the patches with a hammer or vise as if the shears curled them.

Attach patches: Use a woodworking clamp (padded surface is great to not do any more damage, vice grips may work as well) to hold aluminum patch in place. Pick 2 spots to drill a 3/32" hole through the good fiber and the aluminum patch. Screw together using 6-32 screws and washers to help spread the load into the somewhat weak base fiber/plastic. Clamp, 1 drill, 1 screw, move clamp, drill 2nd, screw 2nd, that process keep the patch in its proper place. Screws always go so that they face up (imagining the parts in their original mounted locations) and patches are on the top/inside of the OEM fiber parts.

Drill clearance through holes: Look at an undamaged area to see the appropriate clearance hole size. It is about 1/4" IIRC. If there is enough undamaged fiber material left you may be able to see the location for the clearance hole. These do not have to be too precise in location nor diameter. If not you can mark it with a sharpie when partially assembled. Drill a clearance hole through each aluminum patch. Find all of the damaged spots on all three components, the center section and both left and right triangular pieces and patch them all. Although I did not have to, this procedure could also probably be used if you also have damaged fender liners or bumper cover from some more extreme scraping.

Reassemble: The speed nuts now slip on to the aluminum patches and provide a nice solid reinforced mounting. Make sure the snap on rubber air block seal(s) are properly installed on the center section of the air dam. These insure proper cooling flows (see #25 in image). Should be obvious but the these 3 fiber component all attach above/inside of the bumper cover and wheel well covers. Also the side parts attach below the main center section. Mount all of the speed nuts before reassembling anything. Reattach the wire hangars and finally install all of the black mounting screws.

Yes, the heads of the 6-32 screws protrude ever so slightly between the repaired parts and the bumper cover. However, the slight bow/dimple created is nearly invisible. These parts on my car are now rock solid stable and are perfectly aligned.

Hope this is clear. I know I really should have taken some pictures. I get in my "mode" and just want the job done. Hopefully this will save a few of you a couple hundred bucks (or at least delay it...). Any questions ask away, I'm sure I can help even sans pictures. I'm certainly not going to take anything back apart now...